The present invention relates generally to protection circuits for rechargeable batteries, and more particularly to a protection circuit for preventing potentially destructive reverse currents that may result when a battery is xe2x80x9cplugged backwardsxe2x80x9d (i.e., inserted with a reverse polarity) into a battery charger.
Many portable electronic devices use a rechargeable battery to provide power. These devices include computers, cellular telephones, pagers, radios, and the like. While there are many types of rechargeable batteries used today, including nickel cadmium and nickel metal hydride, lithium ion batteries have become a popular choice. Lithium ion batteries are typically smaller and lighter than other rechargeable battery types while charge capacity is increased.
The charging of lithium ion batteries is conducted in a different manner than the charging of nickel type rechargeable batteries. Generally, nickel-type rechargeable batteries are charged by applying a constant current from a battery charger until the cell reaches a predetermined voltage or temperature. A lithium ion cell, however, uses a different charging process. First, the lithium ion cell is supplied with a current from a battery charger until the cell""s voltage rises above a threshold. Next, the battery charger is held at the threshold until the current of the cell decreases to a predetermined level.
Therefore, the battery charger should be selected (and operated) according to the type of battery that is to be charged, and care taken to ensure proper polarity of connections of the battery to the charger. Failure to do so may result in damage to at least the battery or battery charger.
The present invention is directed to an apparatus and method that prevents potentially destructive reverse currents in latent parasitic semiconductor devices found in a voltage regulator. More specifically, a protection circuit detects that a battery cell has been inserted into the battery charger with a reversed polarity, and prevents unsafe levels of current from flowing through parasitic semiconductor devices that are forward-biased as a result of the reversed polarity.
According to one aspect of the invention, a charge input circuit is arranged to provide a charging signal. An over-voltage protection circuit is coupled to the charging signal. A reverse-voltage protection circuit is arranged to determine when a cell has been inserted into the battery charger with a reversed polarity. The reverse-voltage protection circuit is arranged to prevent the charging signal from causing a parasitic body diode in the over-voltage detection circuit to conduct a potentially destructive reverse current when the reverse-coupled cell is detected.
According to another aspect of the invention, a protection circuit is arranged to compare a voltage of the charging signal to a maximum-allowable reverse-voltage reference. A fault condition is detected when a reverse-voltage is present and the reverse-voltage exceeds the maximum-allowable reverse-voltage. The reverse-voltage is reduced to prevent conduction by parasitic devices that are present in the charge input circuit when the fault condition is detected.
According to another aspect of the invention, the over-voltage protection circuit and the reverse-voltage protection circuit are arranged to control one or more shunt transistors. The shunt transistors are arranged to shunt current from the charging signal to ground whenever the charging signal voltage falls outside of the range defined by the maximum-allowable forward-voltage and the maximum-allowable reverse-voltage.